38 MEGALONYX, 



for muscular attachment. The back of the shaft (Fig. 2) is more convex, and is 

 especially prominent at the middle. The inner margin is, longitudinally concave 

 and obtuse ; and about three inches below the head of the bone it forms an oblong 

 and very rugged tuberosity corresponding to the trochanter minor. The outer 

 margin is nearly straight, and is thin and subacute, except at the extremities and 

 at the middle, in which last position it forms a thick, oblong tuberosity. Its upper 

 end constitutes a huge trochanter major, the antero-external surface of which is 

 convex and rugged, and its posterior surface is excavated into a deep pit. 



The head of the bone is hemispherical and sessile, but comparatively is more 

 prominent than in the Ai. The trochlea (Figs. 1, 3) for the patella is a trans- 

 versely oblong quadrilateral surface, which is concave from side to side and convex 

 downward. It inclines slightly outwardly, and measures four inches in breadth, 

 by nearly two in depth. 



The condyles (Figs. 2, 3) are separated by a wide notch expanding above pos- 

 teriorly, and to the margin of the trochlea inferiorly. They are directed down- 

 ward and backward ; and that external is isolated from the trochlea, while that 

 internal is connected with it only by a narrow isthmus-like prolongation. The 

 articular surface of the outer condyle is pentahedral, is nearly straight transversely, 

 and is convex from before backward; and it inclines towards the other. The 

 internal condyle resembles the segment of a sphere prolonged anteriorly to join the 

 trochlea for the patella. The external condyloid process is formed by the expan- 

 sion of the outer margin of the shaft into a strong tuberosity, the lower face of 

 which is broad and irregular for the attachment of the corresponding lateral liga- 

 ment. The internal condyloid process is a stout tuberosity placed above the con- 

 dyle antero-internally and terminating in a broad surface of attachment for the 

 internal lateral ligament. 



Indies. 



Length of the femur ^Ig 



Breadth obliquely from the head to trochanter major 10 



Breadth at middle of shaft '^ 



Breadth at condyloid processes . . . . 10 



Breadth at articular surfaces of the condyles . . . . . • • 85 



The specimen of a right femur, in the collection of Col. Wailes, has the same 

 form and proportions as those above described, but is smaller. 



Inches. 



Length . . . . . ' . . • • 2O2 



Breadth at middle of shaft . . . . 6 



Tibia.— {VI. XI, Fig. 4 ; PI. XII, Figs. 1, 2, 3, 4.) The tibi^ in Dr. Owen's 

 collection are quite perfect. They are relatively very much shorter than those of 

 recent sloths, but are of greater relative length than those of Mylodon, to which 

 they bear considerable resemblance of form. 



The shaft is prismoid, and rapidly expands towards its extremities. The antero- 

 internal face (PI. XII, Fig. 1) forms a nearly vertical plane, about three inches 



